JPH05153427A - Filter circuit - Google Patents

Abstract

PURPOSE:To minimize the deterioration in S/N by clamping an input signal at the tip of its synchronism, fixing a DC voltage to be constant and inputting the result to the active filter. CONSTITUTION:A clamping circuit 31 is provided between an FM modulation circuit 22 and an active filter circuit 23, an input signal is modulated by the FM modulation circuit 22 and clamped at the tip of the synchronism of a composite video signal including a spurious component to make the DC voltage constant. Thus, even when the input amplitude of the video signal is increased up to the limit of a dynamic range of the active filter 23, no distortion is produced and the capability of the S/N of the active filter circuit 23 is maximized. Thus, even when the degree of the filter is increased to make the filter characteristic steep, since the dynamic range in the active filter circuit 23 is large, the deterioration in the S/N is reduced and the circuit is advantageous to the high circuit integration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a filter circuit,
In particular, the present invention relates to a filter circuit configured on a semiconductor integrated circuit to limit the band of a video signal.

[0002]

2. Description of the Related Art FIG. 5 is a diagram showing a conventional configuration of a filter circuit provided after an FM demodulation circuit for demodulating an FM-modulated video signal. In the figure, 11 is an FM signal input terminal, 21 is an amplitude limiting amplifier circuit for limiting the amplitude of the input signal, 22 is an FM demodulation circuit, and 23 is the FM demodulation circuit 22.
This is an active filter circuit for removing spurious (unnecessary vibration) components output from the semiconductor integrated circuit. Reference numeral 24 is a signal amplifier circuit, and 12 is a demodulated signal output terminal.

Next, the operation will be described. The FM-modulated video signal is input from the FM signal input terminal 11, the signal amplitude is limited to a fixed value in the amplitude limiting amplifier circuit 21, the amplitude noise component is removed, and then the signal is transmitted to the FM demodulation circuit 22. This FM demodulation circuit 22 demodulates the FM signal from which the amplitude noise component has been removed and reproduces the original video signal. However, in the FM demodulation circuit that demodulates a wideband modulated signal like a video signal, A normal demodulation output is combined with a spurious component having a frequency of the input FM signal or a frequency higher than that, and is output. Therefore, in order to obtain the FM demodulated signal, it is necessary to remove spurious components through a filter circuit (low pass filter). That is, the active filter circuit 23 in the subsequent stage is a filter circuit prepared for this purpose, and the video signal from which spurious components have been removed by the active filter circuit 23 is amplified to the required amplitude by the signal amplification circuit 24 in the subsequent stage. And is output to the demodulation signal output terminal 12.

[0004]

The conventional filter circuit is configured as described above, and the active filter circuit formed on the semiconductor integrated circuit is different from the passive filter composed of the coil and the capacitor in the power supply voltage. The dynamic range is limited by the circuit configuration, and the linearity of input / output transmission is deteriorated for an input signal exceeding the dynamic range of the circuit. On the other hand, as shown in FIGS. 6 (a) and 6 (b), the video signal has a feature that the average DC voltage changes depending on the content of the signal, so in order to transmit the video signal without distortion, As shown in 6 (c), a filter circuit having a dynamic range about twice its maximum amplitude is required.

Therefore, in order to transmit a signal without distortion in the active filter circuit, it is necessary to set the output amplitude from the FM demodulation circuit to about 1/2 or less of the dynamic range of the active filter circuit. There was a problem that the dynamic range it had could not be fully utilized.

Further, in the active filter circuit, if the order of the filter is increased to obtain a filter having steeper characteristics in order to enhance the band limiting effect, the number of stages of the circuit increases and the generated noise increases, resulting in a signal-to-noise ratio. Is deteriorated, and especially when the dynamic range of the active filter cannot be fully utilized as described above, the signal-to-noise ratio is deteriorated. Further active filters,
The same problem occurs in the case of using for removing the clock pulse component which becomes the spurious component of the output of the video signal delay circuit using the charge coupled device.

The present invention has been made in order to solve the above problems, and makes maximum use of the signal-to-noise ratio capability of an active filter circuit to limit the band of a video signal with low noise. The object is to obtain a filter circuit that can be used.

[0008]

In the filter circuit according to the present invention, a clamp circuit is provided in the preceding stage of the active filter circuit to clamp the demodulated video signal containing the FM demodulated spurious component at the sync tip, and then to the active filter circuit. It is the one that is input.

Further, a clamp circuit is provided in the preceding stage of the active filter circuit, and the video signal after the delay by the charge-coupled device or before the delay is clamped at the sync tip and then input to the active filter circuit.

[0010]

According to the present invention, since the video signal including the spurious component input to the active filter circuit is clamped at the synchronizing tip, the DC voltage becomes constant regardless of the content of the video signal, which allows the active filter circuit to operate. The input signal amplitude can be increased to near the limit of the input dynamic range.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A filter circuit according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a filter circuit according to an embodiment of the present invention. The same reference numerals as those in FIG. 5 denote the same or corresponding portions, and 31 is provided between an FM demodulation circuit 22 and an active filter circuit 23. 41 is a clamp circuit input terminal, and 41 is a clamp circuit input terminal, and 42 is an output terminal of the active filter 23.

Next, the operation will be described. The FM-modulated video signal input from the FM signal input terminal 11 has its amplitude noise component removed by the amplitude limiting amplification circuit 21 and is modulated by the FM demodulation circuit 22 in the subsequent stage, as in the case of the conventional example of FIG. A demodulated video signal containing spurious components is obtained.

The demodulated video signal containing this spurious signal (see FIGS. 6 (a) and 6 (b)) is input from the clamp circuit input terminal 41 to the clamp circuit 31, and a DC voltage is clamped at the tip of the sync signal of the demodulated video signal. (See FIG. 6 (d)). That is, normally, the signal DC voltage of the demodulated video signal changes depending on the brightness of the video, but the clamp circuit 31
By performing the synchronous front end clamp at, the sync signal of the demodulated video signal and the DC voltage of the pedestal portion become constant regardless of the content of the video.

As a result, the dynamic range required for signal transmission in the active filter can be made substantially equal to the maximum amplitude of the video signal. In other words, distortion does not occur even if the signal is transmitted with the same dynamic range as the maximum amplitude of the video signal.

Then, the clamped demodulated video signal is input to the active filter circuit 23, where spurious components are removed. Since the demodulated video signal input to the active filter 23 does not swing larger than the maximum amplitude of the video signal in terms of direct current, the demodulation sensitivity of the FM demodulation circuit 22 is set to a large value, and the dynamic range of the active filter circuit 23 is barely reached. The input amplitude can be increased.

The demodulated signal from which the spurious component has been removed is transmitted from the active filter output terminal 42 to the signal amplification circuit 24, amplified to a desired amplitude, and output from the demodulated signal output terminal 12. Therefore, the gain of the signal amplifier circuit 24 for obtaining the predetermined output level can be set to be about half as small as that of the conventional example shown in FIG.

As described above, according to the present embodiment, the clamp circuit 31 is provided between the FM demodulation circuit 22 and the active filter circuit 23, and the sync tip of the demodulated video signal modulated by the FM demodulation circuit 22 and containing the spurious component is synchronized. Since the DC voltage is clamped at a constant level, distortion does not occur even if the input amplitude of the video signal is increased up to the dynamic range limit of the active filter 23, and the signal-to-noise ratio of the active filter circuit 23 is maintained. Can be utilized to the maximum extent, and as a result, even if the order of the filter is increased and the filter characteristics are made steep, the deterioration of the signal-to-noise ratio is reduced due to the large dynamic range in the active filter circuit 23, This is advantageous for high integration.

Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, the present invention is applied to an active filter used in a signal delay circuit using a charge coupled device. In the figure, 13 is a video signal input terminal,
Reference numeral 32 is a video signal delay circuit using a charge coupled device, and 42 is an active filter output terminal.

Next, the operation will be described. The video signal input from the video signal input terminal 13 is controlled by the video signal delay circuit 32 including a charge-coupled device and its clock frequency and CC.
It is transmitted to the clamp circuit 31 from the clamp circuit input terminal 41 after being delayed for a predetermined time according to the number of D element stages. Then, after the DC voltage is made constant at the tip of the synchronizing signal by the clamp circuit 31, it is input to the active filter circuit 23, where the clock pulse component generated in the delay circuit 32 is removed and output from the active filter output terminal 42. ..

With this configuration, the clamped video signal is input to the active filter circuit 23. Therefore, the dynamic range of the active filter circuit 23 is fully utilized and the signal-to-noise ratio is optimized. The active filter circuit 23 can be activated.

FIG. 3 shows a modification of the second embodiment of the present invention, in which the video signal before delay is clamped. That is, since the charge coupled device transmits direct current,
In contrast to the configuration of FIG. 2, the connection order of the clamp circuit 31 and the video signal delay circuit 32 is reversed, and the active filter circuit 2
The same effect can be obtained by DC coupling between 3 and the video signal delay circuit 32.

FIG. 4 shows a further modification of the second embodiment of the present invention, which is a video signal delay circuit 3 using a charge coupled device.
2 and the active filter 23 have different optimum input amplitudes due to the dynamic range, and the dynamic range of the video signal delay circuit 32 is small and not optimum as the input of the active filter 23. In the figure, 33 is the video signal delay circuit 32 and the clamp circuit. It is an amplifier circuit connected between 31 and.

That is, the video signal is input to the video signal input terminal 13 with the optimum input amplitude to the video signal delay circuit 32, and the output is amplified by the amplifier circuit 33 so that the active filter 23 has the optimum amplitude. By doing so, it is possible to minimize the deterioration of the signal-to-noise ratio that occurs in the semiconductor circuit in each circuit of the video signal delay circuit 32 and the active filter 23 for band limitation.

The gain of the amplifier circuit 33 is determined so that the respective dynamic ranges of the video signal delay circuit 32 and the clamp circuit 31 have optimum values, so that the gain of the video signal delay circuit 32 and the clamp circuit 31 is increased. Depending on the magnitude relationship of the dynamic range of, it may be a minus, that is, an attenuation circuit.

In each of the above embodiments, the video signal is used as an example of the input signal, but the signal used is not limited to the video signal, and any other signal can be used as long as the DC voltage varies depending on the signal content. May be

[0026]

As described above, according to the filter circuit of the present invention, the input signal is clamped at the synchronizing tip to fix the DC voltage at a constant value and then input to the active filter. The dynamic range of can be used to the maximum, in other words, the amplitude of the video signal input to the active filter can be increased, and as a result, the order of the filter can be increased to obtain a filter having steep characteristics. Even if it is used, there is an effect that the signal to noise ratio can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a filter circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a filter circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a modification of the filter circuit according to the second embodiment of the present invention.

FIG. 4 is a block diagram showing a further modification of the filter circuit according to the second embodiment of the present invention.

FIG. 5 is a block diagram showing a conventional filter circuit.

FIG. 6 is a signal waveform diagram for explaining the operation of the filter circuit according to the related art and the embodiment of the present invention.

[Explanation of symbols]

 22 FM modulation circuit 23 Active filter circuit 24 Signal amplification circuit 31 Clamp circuit 32 Video signal delay circuit

Claims (2)

[Claims] 1. A filter circuit formed on a semiconductor integrated circuit, comprising an active filter for removing an unnecessary vibration component of a wideband signal after modulation, which is modulated by a modulator and whose average DC voltage changes depending on the signal content. In the filter circuit having, a clamp circuit is provided in the preceding stage of the active filter, and the DC voltage of the modulated wideband signal is clamped at each synchronization tip to be constant, and then input to the active filter. Filter circuit to do. 2. A filter circuit having an active filter for removing a clock pulse component of a delayed video signal, which is delayed by using a charge-coupled device and whose average DC voltage changes depending on the signal content, and is clamped before the active filter. A filter circuit, wherein a circuit is provided, and the DC voltage of the delayed or undelayed video signal is clamped at each synchronization tip to be constant and then input to the active filter.